Voltage-controlled variable attenuators have been widely used for automatic gain control circuits as well as for various switches. In broadband microwave amplifiers, they are indispensable for temperature compensation of gain variation.
Historically, variable attenuation was realized by PIN diodes and a pair of hybrid couplers. However, the frequency performance of these attenuators is limited by the bandwidth of the hybrid couplers. More significantly, these attenuators are not compatible with GaAs monolithic circuits because of the PIN diodes.
As a result of this incompatibility, there were developed at least four forms of variable attenuators that are constructed as GaAs monolithic circuits. The first two of these involve the use of FETs (field-effect transistors) in T- or .pi.-configurations, such as is described by Tajima et al. in "GaAs Monolithic Wideband (2-18 GHz) Variable Attenuators", IEEE MTT-S Digest, 1982, pages 479-481. In these basic configurations, however, parasitic capacitance of the FETs degrades the high-frequency performance of an attenuator at both maximum and minimum attenuation states.
Modifications to the basic configurations have therefore been developed to overcome this basic limitation. A modified T-configuration is described by Kondoh in "DC-50 GHz MMIC Variable Attenuator with a 30 dB Dynamic Range", IEEE MTT-S Symposium Digest, 1988, pages 499-502. This configuration requires the use of high impedance lines to separate four FET shunt cells.
An alternative approach, in what is referred to as a bridged-T configuration, is described by Barta et al. in "Surface-Mounted GaAs Active Splitter and Attenuator MMIC's Used in a 1-10-GHz Leveling Loop", IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-34, No. 12, December 1986, pages 1569-1575. This circuit uses a reference (duplicate) cell and op amp to reduce control of two FETs, operated in the linear range, to a single control input.
A third approach has been to use cascaded switchable attenuation circuits that are selectively replaced with a reference path. Such an approach is disclosed by Gupta et al. in "A 0.05- to 14-GHz 5-Bit Digital Attenuator", GaAs IC Symposium, IEEE, 1987, pages 231 to 234. T-networks are used for each digitally selectable segment of the attenuation path. Attenuation ratios for each bit are provided by using different FET gate widths and shunt resistor values.
A fourth approach has been the use of segmented dual-gate MESFETs, as described by Hwang et al. in "A Microwave Phase and Gain Controller With Segmented-Dual-Gate MESFETs in GaAs MMICs", IEEE MTT-S Monolithic Symposium, 1984, pages 1-5. With this approach digital gain control is achieved by properly scaled gate-width ratios among the dual-gate MESFETs.
The various approaches are discussed by Jones in "Digitally Controlled MMIC Attenuators - Techniques and Applications", Military Microwave Conference, 1988, pages 217-222. Jones selects the segmented dual-gate approach for its compactness and reduced input loss.
The minimum number of control bits of a digital attenuator defines the maximum range of attenuation that can be achieved. For example, Gupta uses a 5-bit attenuator design of 8-, 4-, 2-, 1-, and 0.5-dB selectable attenuation components, resulting in 15.5-dB dynamic range and 0.5-dB step resolution. If 1-dB step resolution is used, a 5-bit attenuator can achieve a dynamic range of 31-dB.
These cascaded multi-section methods characteristically have high insertion loss at the minimum attenuation state due to the losses of the reference path associated with each bit. PIN diodes, as mentioned, have lower loss as switching devices than GaAs FETs, but are not compatible with GaAs monolithic process technology. Segmented dual-gate attenuators have lower loss but require the use of switches on the main signal path that create noise, add losses due to parasitic capacitance of the switching devices, have D.C. power dissipation, requires input and output impedance-matching circuitry, and have limited power-handling capability.